input-group-btn use nowrap. Inner btn get inline-block. In that combination, any space character between the button tags take its space. The old workaround try to fix it by using negative margin-left: -4px, but that does not work well, because it dose not depend on the font size. If you did not have a space between button tags and the left button was cropped.

I try a new solution. I set the font-size to 0, so space characters will have no size. And I set margin-left: -1px for siblings, to get 1px border.

Inner btn get set the own font-size, so it should have not negative side effects.

This will fix:
#11431
#9712